Frequency agile exciter

ABSTRACT

Systems and methods are provided for providing high dynamic range operation over a variable range of frequencies. A delta-sigma modulator, having associated frequency characteristics, produces a digital output signal. A digital-to-analog converter converts the digital output signal into an analog signal. A clock circuit provides a clock signal to the delta-sigma modulator and the digital-to-analog converter. A frequency control controls one or more of the clock circuit, the delta-sigma modulator, and the digital-to-analog converter to alter the frequency characteristics of the delta-sigma modulator. A filter circuit can provide one or more passbands to one or more downstream amplifiers ensuring that out of band quantization noise is removed before amplification.

TECHNICAL FIELD

The present invention relates generally to communications systems, andmore specifically to signal transmitters.

BACKGROUND OF THE INVENTION

Efforts in the design of integrated circuits for radio frequency (RF)communication systems generally focus on improving performance, reducingcost or a combination thereof. One area of increasing interest relatesto conversion of signals, such as from analog-to-digital ordigital-to-analog. Both types of conversion have benefited from thedevelopment and use of delta-sigma modulation.

Delta-sigma modulation is a technique used to generate a coarse estimateof a signal using a small number of quantization levels and a very highsampling rate. Limiting a signal to a finite number of levels introduces“quantization” noise into the system. Oversampling and the use of anintegrator feedback-loop in delta-sigma modulation are effective inshifting quantization noise to out-of-band frequencies. The noiseshifting properties enable efficient use of subsequent filtering stagesto remove noise and produce a more precise representation of the input.

The use of delta-sigma modulation in a digital-to-analog conversion canproduce an analog signal having a high dynamic range, but only forlimited ranges of frequency. Outside of a narrow frequency band having ahigh dynamic range, the delta-sigma modulation produces significantquantization noise. In effect, the delta-sigma modulator trades themajority of its spectral range for a few regions of high dynamic rangeoperation. The out-of-band quantization noise produced by thedelta-sigma modulation requires an analog filter to attenuate allfrequencies outside of the narrow low-noise bands.

While delta-sigma modulators have been useful in digital-to-analogconversion applications dealing with a narrow band of frequencies,frequency hopping and multi-frequency applications have required awide-band, convention, digital-to-analog converter to achieve necessaryspectral coverage. The performance of existing digital-to-analogconverters using delta-sigma modulators, referred to hereinafter as adelta-sigma DACs, simply degrades too quickly to be useful outside ofits narrow spectral band. Delta-sigma DACs possess a number of desirablecharacteristics, such as a high dynamic range and the ability todirectly produce radio frequency signals, that would make them usefulfor these applications. The present invention enhances the utility ofdelta-sigma DACs, enabling them to support a wider range of frequenciesand in turn support multi-band and multi-standard transmitters.

SUMMARY OF THE INVENTION

The following presents a simplified summary of the invention in order toprovide a basic understanding of some aspects of the invention. Thissummary is not an extensive overview of the invention. It is intendedneither to identify key or critical elements of the invention nordelineate the scope of the invention. Its sole purpose is to presentsome concepts of the invention in a simplified form as a prelude to themore detailed description that is presented later.

In accordance with one aspect of the present invention, a frequencyagile exciter assembly provides high dynamic range operation over avariable range of frequencies. A delta-sigma modulator, havingassociated frequency characteristics, produces a digital output signal.A digital-to-analog converter converts the digital output signal into ananalog signal. A clock circuit provides a clock signal to thedelta-sigma modulator and the digital-to-analog converter. A frequencycontrol controls one or more of the clock circuit, the delta-sigmamodulator, and the digital-to-analog converter to alter the frequencycharacteristics of the delta-sigma modulator.

In accordance with another aspect of the present invention, a digitalfrequency synthesizer having frequency agility is provided. Adigital-to-analog converter receives a digital input signal and outputsan analog signal having an associated frequency. A tunable filter,having at least one passband with a respective central frequency,filters the analog output signal. A frequency control controls therespective central frequencies of the at least one passband.

In accordance with yet another aspect of the present invention, a methodis provided for providing a frequency agile delta-sigmadigital-to-analog converter. At least one frequency characteristicassociated with a delta-sigma modulator is altered. A tunable filter isconfigured to an appropriate passband according to the frequencycharacteristics of the delta-sigma modulator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a functional block diagram of a transmitter systemcomprising a tunable exciter in accordance with one aspect of theinvention.

FIG. 2 is a graph of an analog output signal in which power is plottedas a function of frequency for the analog output of a frequency-agiledelta-sigma DAC implemented in accordance with an aspect of the presentinvention.

FIG. 3 illustrates a functional block diagram of an exemplary embodimentof an tunable exciter in accordance with an aspect of the presentinvention.

FIG. 4 illustrates a functional block diagram of another exemplaryembodiment of an exciter in accordance with another aspect of thepresent invention.

FIG. 5 illustrates a multi-carrier exciter in accordance with one ormore aspects of the present invention.

FIG. 6 illustrates an exemplary frequency agile digital synthesizersystem in accordance with an aspect of the present invention.

FIG. 7 illustrates another exemplary frequency agile digital synthesizersystem in accordance with another aspect of the present invention.

FIG. 8 illustrates a methodology for varying frequency characteristicsof an exciter in accordance with one or more aspects of the presentinvention.

DETAILED DESCRIPTION OF INVENTION

The present invention relates to systems and methods for varyingfrequency characteristics of an exciter. In accordance with one or moreaspects of the invention, the exciter can include a digital-to-analogconverter (DAC) having one or more variable frequency characteristics.For example, the center frequency and shape of one or more regions ofhigh-dynamic range associated with the DAC can be altered at a centralfrequency control to allow for changing input frequencies. In anexemplary embodiment, the DAC is a delta-sigma DAC (e.g., a DAC thatoperates on a delta-sigma modulated signal). In accordance with anaspect of the invention, the exciter can further include a tunablefilter for attenuating noise outside of a desired range of frequencies.The methods and systems described herein are generally useful inexciters, but are also applicable to multi-carrier applications andwithin the field of direct digital synthesis.

FIG. 1 illustrates a transmitter system 10 comprising a tunable exciter12 in accordance with one aspect of the invention. The exciter 12receives one or more digital information signals at a delta-sigmamodulator 14. The signals are input into the delta-sigma modulator 14 asa multi-bit input stream at a first sample rate. The delta-sigmamodulator 14 converts the multi-bit input stream into an output streamat a second sample rate. The second sample rate can be selectedaccording to desired frequency characteristics of an analog outputsignal associated with the system.

The output stream is provided to a digital-to-analog converter (DAC) 16.The digital-to-analog converter 16 converts the digital output streaminto one or more analog signals, each having a characteristic frequencyassociated with the second sample rate. In an exemplary embodiment, thecharacteristic frequency is a radio frequency, such that the analogsignal can be broadcast without any change in frequency. By radiofrequency, it is intended to encompass the range of feasibletransmission frequencies, including both traditional radio frequency(RF) ranges (e.g., megahertz range) and microwave frequency ranges(e.g., gigahertz range). The characteristic frequency can, however, bean intermediate frequency, requiring one or more upconverters (notshown) within the exciter to convert the intermediate frequency signalto an appropriate frequency for transmission.

It will be appreciated that the output of the delta-sigma modulator cancontain a considerable amount of quantization noise. For example, adelta-sigma modulator transforming a twelve-bit output into a one-bitoutput will generate a large amount of quantization noise. One of thepurposes of the delta-sigma modulator 14 is to shape the noise in adesired fashion. For example, much of the noise can be shifted tofrequencies well outside a narrow frequency band of interest. Once thenoise has been shifted away from the narrow frequency band of interest,it can be removed from the signal without undue impact on the frequencyband of interest.

To this end, the analog signals are then provided to a tunable filter18. The tunable filter 18 has one or more characteristic passbands withassociated center frequencies. The filter allows signals having afrequency falling within the passbands to pass through in approximatelytheir original state, but attenuates signals having a frequency fallingoutside of the passbands. It will be appreciated that one or more of thepassbands of the tunable filter 18 can be arranged to coincide with thefrequency band of interest for the delta-sigma modulator 14 and DAC 16.Thus, the out-of-band quantization noise can be filtered from thesignal, and the filtered signal can be provided to an amplifier 20 fortransmission.

A problematic characteristic of delta-sigma modulation is the narrowrange of frequencies produced by a delta-sigma modulator. The quality ofa delta-sigma modulated signal degrades quickly as it diverges from anarrow, high dynamic range band associated with the delta-sigmamodulator. Thus, while the delta-sigma provides a signal having a highdynamic range over its associated band, it is effectively limited tothis narrow band of frequencies.

In accordance with an aspect of the present invention, each of thedelta-sigma modulator 14, the DAC 16, and the tunable filter 18 can becontrolled by a frequency control 22. The frequency control 22 isoperative to change one or more frequency characteristics of at leastone of the delta-sigma modulator 14, the DAC 16, and the tunable filter18 to provide a frequency agile exciter 12. For example, the frequencycontrol can alter a characteristic center frequency or shape of a highdynamic range frequency band associated with the delta-sigma modulator14. The frequency control 22 can comprise a microprocessor runningcustomized software, a specialized digital or analog signal processor,or any of a number of other signal evaluation components. The frequencycontrol 22 can receive input from a user as configuration input orfrequency information can be passed to the frequency control fromcomponents upstream of the delta-sigma modulator (not shown).

The frequency control 22 allows the exciter 12 to dynamically adjust toa number of different frequencies by adjusting the frequency band of oneor more components. For example, the frequency control can change clockrates associated with the delta-sigma modulator 14 and the DAC 16 tochange the frequency characteristics of the one or more high dynamicrange frequency bands associated with the delta-sigma modulator 14 andthe DAC 16. The passband frequencies of the tunable filter 18 can alsobe changed by the frequency control 22 to match a desired outputfrequency of the DAC 16. These changes can occur in real time, allowingthe exciter 12 to be used for frequency hopping and frequency divisionschemes.

FIG. 2 is a graph 30 of an analog output signal in which power (dBm) isplotted as a function of frequency (MHz) for the analog output of afrequency-agile delta-sigma DAC implemented in accordance with an aspectof the present invention. The signal is shown at a first time as a firstline 32 drawn in black and at a second time as a second line 34 drawn ingray. As mentioned previously, a delta-sigma DAC according to an aspectof the present invention can provide a very low noise region (e.g., 36)of the spectrum, which is particularly useful for wirelesscommunications applications. By providing for frequency agile operationof the delta-sigma DAC, a delta-sigma DAC can support multi-band andmulti-standard transmitters.

At a first time, corresponding to the first line 32, the signal isoperating in a GSM band. For this example, a low noise region 36associated with the analog signal is centered at about 940 MHz. The lownoise region 36 extends for a bandwidth ranging from about 30 MHz toabout 100 MHz around the center frequency. In accordance with an aspectof the present invention, the center frequency of the signal can bechanged, such that the signal at a second time, corresponding to thesecond line 34, is operating in a WCDMA frequency band. Thus, a lownoise region 38 associated with the signal is now centered at about 2135MHz. For the purpose of example, only the center frequency of thepassband has been changed in the illustrated graph. It will beappreciated, however, that the width and power of the low noise regioncan also be changed in accordance with the present invention.

FIG. 3 illustrates a functional block diagram of an exemplary tunableexciter 50 in accordance with an aspect of the present invention. Theexciter 50 comprises a delta-sigma modulator 52 that receives a digitalinput signal at a first word size and sample rate, and outputs amodulated output signal having a second word size and sample rate. Inthe illustrated example, the first word size is twelve bits and thesecond word size is one bit. It will be appreciated that the digitalinput signal can be oversampled to produce an output signal at anincreased rate, or frequency. For example, the delta-sigma modulator 52and a digital-to-analog converter 90 can process the oversampled inputsignal at a high rate to directly produce radio frequency output withoutthe need for upconversion of the signal.

The delta-sigma modulator 52 can comprise one or more stages, limitedonly by practical considerations. The delta-sigma modulator 52 quantizesthe input signal, but in such a way as to shape the quantization noiseproduced by the delta-sigma modulator 52 into frequencies outside of aband of interest. It will be appreciated that even though two stages areillustrated and described, the delta-sigma modulator 52 can comprisemore than two stages or a single stage. The number of stages and thefilter coefficients determine the number and location of zeroes in thenoise transfer function; in turn determining the shape and width of thelow noise region.

A first stage comprises a first summer 54 and a first integrator 56. Thefirst summer 54 receives an input signal and weighted feedback from theoutput of a second stage. The feedback signal is weighted by a scalarvalue at a first register 58. It will be appreciated that the scalarvalue associated with the first register 58, and any subsequentregisters can be negative, such that the feedback signal is deductedfrom the input signal. The first summer 54 sums the received signals andpasses the sum to the first integrator 56. The output of the firstintegrator 56 provides the input for a second stage.

The second stage comprises a second summer 60 and a second integrator62. The output of the first integrator 56 is received at the secondsummer 60. The second summer 60 also receives a weighted, quantized,feedback from a quantizer 66. The feedback signal is weighted by ascalar value at a second register 64. The summed signals are passed tothe second integrator 62. The output of the second integrator is passedto the quantizer 66, which converts the output of the integrator into apredetermined number of digital bits. In the illustrated embodiment, theoutput of the quantizer has a smaller digital word size than the inputof the first summer 52.

The quantizer 66 is driven by a clock circuit 86 to produce its outputat a selected sample rate. In an exemplary embodiment, the clock circuitcan be a digital frequency synthesizer. The clock circuit 86 iscontrolled by a frequency control 88 that drives the clock circuit 86 toachieve a desired clock rate for the delta-sigma modulator 52 associatedwith a desired output frequency for the exciter 50. The frequencycontrol can be made responsive to user input.

A delta-sigma modulator 52 provides one or more narrow high dynamicrange frequency bands by shifting its associated quantization noise toout-of-band frequencies. The high dynamic range frequency bandsassociated with delta-sigma modulator 52 generally have centerfrequencies that are multiples of its clock rate. In an exemplarydelta-sigma modulator, a center frequency of one high dynamic range bandcan be found at one-fourth of the clock rate. Thus, the position ofthese high dynamic range bands can be shifted across frequencies bychanging the associated clock rate of the delta-sigma modulator 52.

The output of the quantizer 66 is then provided to a digital-to-analogconverter 90. The digital-to-analog converter 90 is also driven by theclock circuit 86, such that it operates at the same rate as thedelta-sigma modulator 52. The output of the digital-to-analog converter90 is provided to a tunable filter 92. The tunable filter 92 attenuatessignals having a frequency outside one or more passbands associated withthe filter. Each of the filter passbands has an associated centerfrequency that can be altered in response to a control signal from thefrequency control 88. This allows the system to react to changes in thefrequency of the analog output from the digital-to-analog converter 90by shifting one or more passbands.

The tunable filter 92 can comprise a bank of filters, each having adesired passband of interest. Alternatively, the tunable filter 92 cancomprise a surface acoustic wave (SAW) filter that can electronicallycontrolled to configure one or more micromechanical components thatdefine its one or more associated passband frequencies. Other tunablefilters having similar frequency agility can be utilized in accordancewith one or more aspects of the invention. The output of the tunablefilter 92 is a clean analog signal at a desired frequency that can beamplified and broadcast according to know methods.

FIG. 4 illustrates an exemplary embodiment of an exciter 100 inaccordance with another aspect of the present invention. The exciter 100comprises a delta-sigma modulator 102 that receives a digital inputsignal at a first word size and sample rate and outputs a modulatedoutput signal having a second word size and sample rate. In theillustrated example, the first word size is twelve bits and the secondword size is one bit. It will be appreciated that the digital inputsignal can be oversampled to produce an output signal at an increasedrate, or frequency. For example, the delta-sigma modulator 102 and anassociated digital-to-analog converter 138 can process the oversampledinput at a high rate to directly produce radio frequency output withoutthe need for upconversion of the signal.

The delta-sigma modulator 102 comprises one or more stages, limited onlyby practical considerations. The delta-sigma modulator 102 quantizes theinput signal, but in such a way as to shape the quantization noiseproduced by the delta-sigma modulator 102 into frequencies outside of aband of interest. It will be appreciated that even though two stages areillustrated and described, the delta-sigma modulator 102 can comprisemore than two stages or a single stage.

A first stage comprises a first summer 104 and a first integrator 106.The first summer 104 receives an input signal and weighted feedback fromthe output of a second stage. The feedback signal is weighted by ascalar value at a first register 108. It will be appreciated that thescalar value associated with the first register 108, and any subsequentregisters can be negative, such that the feedback signal is deductedfrom the input signal. The first summer 104 sums the received signalsand passes the sum to the first integrator 106. The output of the firstintegrator 104 provides the input for a second stage.

The second stage comprises a second summer 110 and a second integrator112. The output of the first integrator 106 is received at a secondsummer 110. The second summer 110 also receives a weighted, quantized,feedback from a quantizer 116. The feedback signal is weighted by ascalar value at a second register 114. The summed signals are passed tothe second integrator 112. The output of the second integrator is passedto the quantizer 116, which converts the output of the integrator into apredetermined number of digital bits. In an exemplary embodiment, thequantizer 116 performs a threshold determination to produce a one-bitoutput for the delta-sigma modulator 102. It will be appreciated,however, that the quantizer 116 can produce a multi-bit output inaccordance with one or more aspects of the present invention.

A frequency associated with the digital output of the delta-sigmamodulator can be altered by a frequency control 136. In accordance withan aspect of the present invention, the register at each stage (e.g, 108and 114) is programmable such that the scalar value associated with theregister can be altered by a control signal from the frequency control.In the illustrated delta-sigma modulator, the feedback registersessentially act as digital filter coefficients. Altering thesecoefficients directly shifts the frequency characteristic of the highdynamic range bands created by the delta-sigma modulator 102 by changingthe “shape” of the quantization noise across the frequency spectrum.Thus, the width and dynamic range of a particular band can be changed byshifting the number of and the placement of one or more zeroes in thefilter transfer function of the delta-sigma modulator. For example, theprogrammable coefficients can be altered to increase the dynamic rangeof a particular high dynamic range region by overlapping multiple lownoise regions at a particular frequency.

The digital output of the delta-sigma modulator 102 is provided to adigital-to-analog converter 138, which converts the output into ananalog signal. This analog signal is provided to a tunable filter 140.The tunable filter can comprise a filter bank, a micromechanicallytunable SAW filter, or any other tunable filter known in the art. Thetunable filter 140 is controllable by the frequency control 136 to adoptone or more passbands having center frequencies corresponding with thelow noise bands of the delta-sigma modulator 102.

It will be appreciated that the various aspects of the inventionillustrated in FIGS. 3 and 4 can be practiced in concert, as well asindependently, to create a delta-sigma digital-to-analog converterhaving passbands with both a programmable center frequency and adaptable“shape” (e.g., passband width and dynamic range).

FIG. 5 illustrates a transmitter system 150 comprising a multi-carrierexciter 151 in accordance with one or more aspects of the presentinvention. A digital signal source 152 provides a plurality of digitalsignals to a delta-sigma modulator 154. Each of the plurality of digitalsignals can have an associated frequency. In an exemplary embodiment,each of the associated frequencies can be found within a relativelynarrow band of frequencies. The digital signal source 152 can be adirect digital synthesis device or any other appropriate digital signalprocessing circuit. The signal is filtered and quantized at thedelta-sigma modulator 154 to bring the digital signals to respectiveradio frequencies. The respective radio frequency associated with eachsignal is determined by the spacing of the original associatedfrequencies. The DSM provides an average output frequency based on itsarchitecture and clock rate. The delta-sigma modulator 154 can befrequency agile in accordance with one or more aspects of the inventionsuch that one or more frequency characteristics of the delta-sigmamodulator are controlled at a frequency control 156.

The output of the delta-sigma modulator 154 is provided to adigital-to-analog converter (DAC) 158. The digital-to-analog converter158 converts the radio frequency digital signals from the delta-sigmamodulator 154 into analog signals. The digital-to-analog converter canbe programmable, in accordance with one or more aspects of the presentinvention, such that the frequency characteristics of the device can bealtered at the frequency control 156. In an exemplary implementation ofthe invention, the output of the delta-sigma modulator 154 is a one-bitoutput, and the digital-to-analog converter 158 has one-bit ofresolution. It will be appreciated, however, that both the output of thedelta-sigma modulator 154 and the resolution of the digital-to-analogconverter can be multi-bit in accordance with one or more aspects of thepresent invention.

The analog signals are provided to a channelizing filter 160, where theyare filtered and separated into a plurality of individual carriersignals with respective radio frequencies. The channelizing filter 160can have a plurality of narrow passbands, each having a center frequencyassociated with the respective radio frequencies of the analog signals.The channelizing filter 160 can be tunable, such that the centerfrequency or shape of the narrow passbands can be altered by thefrequency control 156. In an exemplary implementation, the channelizingfilter 160 is a surface acoustic wave filter that is electricallytunable via one or more micromechanical components within the filter. Itwill be appreciated, however, that any appropriate tunable filter orfilter bank can be used in accordance with one or more aspects of thepresent invention.

The plurality of carrier signals are then provided to a plurality ofamplifiers 162-168. In an exemplary embodiment, each of the plurality ofcarrier signals is provided to a different amplifier, but it will beappreciated that plural sets of signals can be provided to multi-carrieramplifiers. The amplifiers 162-168 amplify the provided signals and theamplified signals are provided to respective antennas (not shown) fortransmission.

FIG. 6 is an example of a frequency agile digital synthesizer system 300in accordance with an aspect of the present invention. The system 300includes a memory 302 that stores N predetermined signal patterns 304,306, and 308, where N is an integer greater than 1. The signal patterns304-308 can be stored as digital representations of corresponding analogsignal patterns that have been generated for a predetermined duration atdesired frequencies. Alternatively, the signals can be generateddigitally, such as with a baseband modulator, DSP or other signalgenerator device. The frequency patterns 304-308 can be spaced over adesired frequency range, such as at fixed or variable intervals betweenadjacent frequencies. It is to be understood and appreciated that anyset of frequencies, which can be related or unrelated, can be stored inthe memory 302 for use in the system 300 according to an aspect of thepresent invention.

A switch system 310, such as a multiplexer, is associated with thememory 302 for selecting a desired one of the N frequency patterns304-308. A frequency control 312 provides a selection signal forcontrolling which of the N patterns is to be provided to an associateddigital-to-analog converter (DAC) 318. The selected pattern can beprovided as an M-bit data stream, where M≧1. Those skilled in the artwill understand and appreciate various types of frequency selectionmechanisms that can be utilized to select a desired carrier frequency inaccordance with an aspect of the present invention. For example, thefrequency control function can be implemented as hardware, software or acombination thereof. In one particular example, the frequency selectioncan be computer-executable instructions implemented in the memory 302.

The DAC 318 converts the data stream into a corresponding analog signalhaving a desired frequency as defined by the selected pattern. The DAC318 can also be controlled by the frequency control 312 to alter theshape or center frequency of a characteristic passband associated withthe DAC 318. Where the system 300 is implemented within transmittercircuitry, the DAC 318 can provide the analog signal directly at adesired transmission frequency according to an aspect of the presentinvention. In this way, no additional analog upconversion is requiredprior to transmission of the filtered signal, and the system can providea substantially pure carrier signal that can be mixed with desired datafor subsequent transmission. Alternatively, the system 300 could beimplemented in conjunction with receiver circuitry or other devicesrequiring substantially pure signals that can hop to desired frequenciesaccording to a hop rate for such applications.

The DAC 318 provides the analog output signal to a tunable filter 320.The tunable filter 320 attenuates analog signals having a frequencyoutside of one or more characteristic passbands. The tunable filter isprogrammable, such that one or more frequency characteristics of itspassbands can be electrically altered by the frequency control 312. Forexample, the frequency control can change associated center frequenciesof one or more passbands of the filter. In an exemplary implementation,the tunable filter 320 is a surface acoustic wave filter that iselectrically tunable via one or more micromechanical components withinthe filter. It will be appreciated, however, that any appropriatetunable filter or filter bank can be used in accordance with one or moreaspects of the present invention.

The filtered signal is provided to an amplifier 322 operative to amplifythe selected signal pattern to provide an output signal at a desiredlevel. For example, the amplified signal can be provided as a localoscillator signal to drive a mixer such that the signal operates as acarrier frequency for transmission of wireless communication signals orfor up or down conversion in a transceiver.

FIG. 7 is an example of another digital synthesizer system 350 inaccordance with an aspect of the present invention. The system 350 issimilar to the system 300 shown and described with respect to FIG. 6,but makes use of a delta-sigma modulator within the digital-to-analogconversion. The system 350 includes a memory 352 that stores Npredetermined signal patterns 354, 356, and 358, where N≧1. The signalpatterns 354-358 can be stored as digital representations ofcorresponding analog signal patterns that have been generated for apredetermined duration at desired frequencies. Alternatively, thesignals can be generated digitally, such as with a baseband modulator,DSP or other signal generator device. The frequency patterns 354-358 canbe spaced over a desired frequency range, such as at fixed or variableintervals between adjacent frequencies. It is to be understood andappreciated that any set of frequencies, which can be related orunrelated, can be stored in the memory 352 for use in the system 350according to an aspect of the present invention.

A switch system 360, such as a multiplexer, is associated with thememory 352 for selecting a desired one of the N frequency patterns354-358. A frequency control 362 provides a selection signal forcontrolling which of the N patterns is to be provided to an associateddelta-sigma modulator 366. The selected pattern can be provided as anM-bit data stream, where M≧1. Those skilled in the art will understandand appreciate various types of frequency selection mechanisms that canbe utilized to select a desired carrier frequency in accordance with anaspect of the present invention. For example, the frequency control 362can be implemented as hardware, software or a combination thereof.

The delta-sigma modulator 366 is programmed and/or configured to processa selected one of the signal patterns to provide a high dynamic rangequantized data stream having a predetermined number of bits for drivingan associated DAC 368 at a desired sample rate. The output data streammay be oversampled to provide a higher dynamic range. Those skilled inthe art will understand and appreciate various approaches that can beutilized to implement a digital delta-sigma modulation in accordancewith an aspect of the present invention. The delta-sigma modulator 366provides a quantized output signal that typically has a reduced numberof bits per sample when compared to the input signal, but at a fastersample rate than the input signal.

In accordance with one or more aspects of the present invention, thedelta-sigma modulator 366 can be controlled by the frequency control 362to change one or more frequency characteristics of the delta-sigmamodulator. For example, delta-sigma modulators tend to shapequantization noise associated with the modulation process as to leave afew narrow bands of high dynamic range operation. The frequency control362 can alter this shaping process to change one or more of the width,center frequency, and the associated dynamic range of the high dynamicrange frequency bands. The process of adapting the delta-sigma modulatorin this manner is described in more detail in the discussion of FIGS. 3and 4 above.

The DAC 368 converts the delta-sigma modulated data stream into acorresponding analog signal having a desired frequency as defined by theselected pattern. The DAC 368 can be a one-bit or multi-bit DAC. Aone-bit or low multi-bit DAC for converting a data stream can facilitateproviding the analog output at a desired frequency, such as can be inthe GHz or upper MHz range. For example, the DAC 368, delta-sigmamodulator 366 and switch system can be configured from SiGe, InP orother high-speed integrated circuit technologies. The delta-sigmamodulator 366 is configured to provide a data stream having anappropriate number of bits according to the DAC 368 being utilized.

The DAC 368 provides the analog output signal having the desiredfrequency to a tunable filter 370. In an exemplary embodiment, the DAC368 can be configured by the frequency control 362 to operate at adesired frequency, such that an associated clock rate of the DAC 368 canbe matched to the varying sample rate of a frequency delta-sigmamodulator. Where the system 350 is implemented within transmittercircuitry, the DAC 368 can provide the analog signal directly at adesired transmission frequency according to an aspect of the presentinvention. In this way, no additional analog upconversion is requiredprior to transmission of the filtered signal, and the system can providea substantially pure carrier signal that can be mixed with desired datafor subsequent transmission. Alternatively, the system 350 could beimplemented in conjunction with receiver circuitry or other devicesrequiring substantially pure signals that can hop to desired frequenciesaccording to a hop rate for such applications.

The tunable filter 370 attenuates analog signals having a frequencyoutside of one or more characteristic passbands. The tunable filter isprogrammable, such that one or more frequency characteristics of itspassbands can be electrically altered by the frequency control 362. Forexample, the frequency control can change associated center frequenciesof one or more passbands of the filter. In an exemplary implementation,the tunable filter 370 is a surface acoustic wave filter that iselectrically tunable via one or more micromechanical components withinthe filter. It will be appreciated, however, that any appropriatetunable filter or filter bank can be used in accordance with one or moreaspects of the present invention.

The filtered output signal can be passed to an amplifier 372 operativeto amplify the selected signal pattern to provide an output signal 374at a desired level. For example, the amplified signal can be provided asa local oscillator signal to drive a mixer such that the signal operatesas a carrier frequency for transmission of wireless communicationsignals or for up or down conversion in a transceiver.

In view of the examples shown and described above, a methodology inaccordance with the present invention will be better appreciated withreference to the flow diagram of FIG. 8. While, for purposes ofsimplicity of explanation, the methodology is shown and described asexecuting serially, it is to be understood and appreciated that thepresent invention is not limited by the order shown, as some aspectsmay, in accordance with the present invention, occur in different ordersand/or concurrently from that shown and described herein. Moreover, notall features shown or described may be needed to implement a methodologyin accordance with the present invention. Additionally, suchmethodologies can be implemented in hardware (e.g., one or moreintegrated circuits), software (e.g., running on a DSP or ASIC) or acombination of hardware and software.

FIG. 8 illustrates a methodology 400 for varying frequencycharacteristics of an exciter in accordance with one or more aspects ofthe present invention. The methodology begins at 402, where the systemawaits an input to a frequency control. This input can originate fromany of a number of sources, including, for example, configuration datafrom a user or an automated configuration message generated by signalprocessing components (not shown) upstream of the exciter. When theinput is received, the system determines at 404 if it is necessary tochange the center frequency of a digital-to-analog conversion (DAC)assembly associated with the exciter. If so, the clock rate associatedwith a delta-sigma modulator within the DAC conversion assembly isaltered at 406 to produce a desired center frequency for one or morehigh dynamic noise regions associated with the DAC assembly. At 408, theclock rate associated with a digital-to-analog converter componentwithin the DAC assembly is changed to match the clock rate of thedelta-sigma modulator. The methodology then advances to 410.

If no change to the center frequency is necessary, the methodologyproceeds directly to step 410. At 410, the system determines if it isnecessary to change the shape of the one or more high dynamic rangeregions associated with the DAC assembly. For example, the width andeffective dynamic range of the regions can be altered. If so, themethodology advances to 412, where one or more feedback registers withinthe delta-sigma modulator are altered to produce the desired shape. Themethodology then advances to 414. If it is not necessary to change theshape of the high dynamic range regions, the methodology advancesdirectly to 414.

At 414, an analog filter on the exciter is configured such that one ormore passbands on the filter coincide with the desired high dynamicrange region of the DAC assembly. This can include changing the centerfrequency of the filter passbands as well as changing their effectivewidth. At 416, a tunable channelizer on the exciter is configured tomatch the desired high dynamic range of the DAC assembly. Once thefilter and channelizer passbands have been configured, the methodologyreturns to 402 to await further configuration input at the frequencycontrol.

What has been described above includes exemplary implementations of thepresent invention. It is, of course, not possible to describe everyconceivable combination of components or methodologies for purposes ofdescribing the present invention, but one of ordinary skill in the artwill recognize that many further combinations and permutations of thepresent invention are possible. Accordingly, the present invention isintended to embrace all such alterations, modifications and variationsthat fall within the spirit and scope of the appended claims.

1. A frequency agile exciter assembly, comprising: a delta-sigmamodulator, having associated frequency characteristics, that produces adigital output signal; a digital-to-analog converter that converts thedigital output signal into an analog signal; a clock circuit thatprovides a clock signal to the delta-sigma modulator and thedigital-to-analog converter; and a frequency control that controls atleast one of the clock circuit, the delta-sigma modulator, and thedigital-to-analog converter to alter the frequency characteristics ofthe delta-sigma modulator.
 2. The assembly of claim 1, the frequencycontrol controlling the clock circuit to alter respective centerfrequencies of at least one high dynamic range region associated withthe delta-sigma modulator.
 3. The assembly of claim 2, the delta-sigmamodulator comprising at least one feedback register and the frequencycontrol being operative to alter at least one scalar value associatedwith the at least one feedback register to change respective widths ofat least one high dynamic range region associated with the delta-sigmamodulator.
 4. The assembly of claim 2, the delta-sigma modulatorcomprising at least one feedback register and the frequency controlbeing operative to alter at least one scalar value associated with theat least one feedback register to change respective dynamic ranges of atleast one high dynamic range region associated with the delta-sigmamodulator.
 5. The assembly of claim 1, the delta-sigma modulatorcomprising at least one feedback register and the frequency controlbeing operative to alter at least one scalar value associated with theat least one feedback register to change respective widths of at leastone high dynamic range region associated with the delta-sigma modulator.6. The assembly of claim 1, the delta-sigma modulator comprising atleast one feedback register and the frequency control being operative toalter at least one scalar value associated with the at least onefeedback register to change respective dynamic ranges of at least onehigh dynamic range region associated with the delta-sigma modulator. 7.The assembly of claim 1, the delta-sigma modulator outputting a one-bitdigital signal.
 8. The assembly of claim 1, further comprising a tunablefilter having at least one passband having a center frequency, thefrequency control being operative to vary the center frequency of the atleast one passband.
 9. The assembly of claim 8, the tunable filtercomprising a surface acoustic wave (SAW) filter.
 10. The assembly ofclaim 8, the tunable filter comprising at least one micromechanicalstructure that can be electrically configured to change the centerfrequency of the at least one passband associated with the filter. 11.The assembly of claim 1, the analog signal being a radio frequencysignal.
 12. The assembly of claim 1, the analog signal comprising aplurality of analog signals having respective associated frequencies,the assembly further comprising a channelizing filter that separates andfilters the plurality of analog signals.
 13. The assembly of claim 12,the frequency control being operative to vary the center frequencies ofa plurality of passbands associated with the channelizing filter. 14.The assembly of claim 13, the channelizing filter comprising a surfaceacoustic wave filter comprising at least one micromechanical structurethat can be electrically configured to change the center frequencies ofplurality of passbands associated with the filter.
 15. A digitalfrequency synthesizer having frequency agility, comprising: adigital-to-analog converter that receives a digital input signal andoutputs an analog signal having an associated frequency; a tunablefilter, having at least one passband with a respective centralfrequency, that filters the analog output signal; and a frequencycontrol that is operative to alter the respective central frequencies ofthe at least one passband and one or more frequency characteristics ofthe digital-to-analog converter.
 16. The digital frequency synthesizerof claim 15, the frequency control being operative to alter one or morefrequency characteristics of the digital-to-analog converter.
 17. Thedigital frequency synthesizer of claim 16, the tunable filter being asurface acoustic wave filter.
 18. The digital frequency synthesizer ofclaim 17, the surface acoustic wave filter being electrically tunablevia micromechanical structures within the filter.
 19. The digitalfrequency synthesizer of claim 15, the digital input signal being adelta-sigma modulated digital signal.
 20. A method of providing afrequency agile delta-sigma digital-to-analog converter (DAC),comprising: altering at least one frequency characteristic associatedwith a delta-sigma modulator; and configuring a tunable filter to anappropriate passband according to the frequency characteristics of thedelta-sigma modulator.
 21. The method of claim 20, the altering of atleast one frequency characteristic including changing at least oneprogrammable register within the DAC.
 22. The method of claim 20, thealtering of at least one frequency characteristic comprising altering aclock rate associated with the delta-sigma modulator and the methodfurther comprising altering a clock rate associated with adigital-to-analog converter to match the clock rate of the delta-sigmamodulator.
 23. The method of claim 20, the altering at least onefrequency characteristic further comprising converting the digitaloutput of the delta-sigma modulator into an analog signal and summingthe analog signal with a delayed representation of the analog signal.24. The method of claim 20, the configuring of the tunable filterincluding electrically changing micromechanical structures within thefilter.
 25. A frequency agile exciter, comprising: means for convertinga signal from a digital signal to an analog signal, the means havingassociated frequency characteristics; means for altering the associatedfrequency characteristics of the means for converting.
 26. The exciterof claim 25, further comprising means for filtering the analog signal,the means having associated frequency characteristics.
 27. The exciterof claim 26, the means for altering being operative to alter thefrequency characteristics of the means for filtering.